Temperature differences between the top and the base of pipelines can, in specific circumstances, drive severe corrosion due to condensed fluids at the top-of-the-line. Here, we measure corrosion at the top-of-the-line in the condensate phase and, simultaneously, in the immersed phase at the bottom-ofthe-line using an electrochemical approach. With CO 2 -saturated chloride brine, corrosion at the top-of-the-line increased with fluid temperature (due to increased condensation rates), whereas at the bottom-of-the-line, the corrosion rate decreased (due to ferrous carbonate precipitation). The addition of acetic acid increased the corrosion rate at both top and bottom of the line, with any ferrous carbonate films being removed by dissolution. The addition of methyl diethanolamine (MDEA) reduced corrosion rates significantly because of the increased pH in both the condensate at the top-of-the-line and under immersion at the bottom-of-the-line, reducing the solubility of FeCO 3 . Overall, the addition of acetic acid was confirmed to damage and remove protective FeCO 3 films. However, continuous films were not re-established after MDEA additions, rather porous and generally unprotective films were formed.

Carbon steel flowlines transporting hydrocarbon fluids are susceptible to internal corrosion in the aqueous phase at the base (6 o'clock position) and in aqueous condensed droplets at the top (12 o'clock position). Respectively, these issues are known as “bottom‐of‐the‐line” corrosion and “top‐of‐the‐line” corrosion and inhibitors that are used to control internal pipeline corrosion need to be effective at both locations. Here, we explore whether 2‐mercaptobenzimidazole (2‐MBI), 2‐phenyl‐2‐imidazoline (2‐PI), 2‐amino‐5‐ethyl‐1,3,4‐thiodiazole (2‐AETD), are able to control CO2 corrosion simultaneously at both the top‐ and the bottom‐of‐the‐line in the presence and absence of acetic acid, a common minor constituent of produced hydrocarbon fluids. The performance of the species varied between highly effective (2‐MBI), moderately effective (2‐AETD) to ineffective (2‐PI). Inhibition was effective at both bottom‐ and top‐of‐the‐line, and with acetic acid present. Given that the vapor pressure of these species is negligible, it is suggested that they are carried from the bulk phase to the top‐of‐the‐line dissolved in aerosol droplets rather than in the vapor phase.

The integrity of pipelines transporting wet gas has been a concern to oil and gas industry in recent times. Top of the line corrosion (TLC) in particular is important because of the inability to protect the upper wall of pipelines (i.e. in the vapour space) with conventional corrosion inhibitors. Organic acids, such as acetic, are volatile components in oil and gas reservoirs and play significant roles in determining the corrosion process that occurs at the 10-2 o'clock position of the pipelines. TLC of a typical carbon steel pipeline containing 1% NaCl (containing acetic acid) saturated with CO2 gas at 60°C for a period of 100hrs was investigated by applying electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and linear polarisation resistance to mild steel samples exposed to these conditions. Both CO2 and acetic acid are transported in the gas phase and condensed with water thereby contributed to corrosion processes at the top of the line. Preliminary results demonstrated a progressive reduction in corrosion rate with time in the CO2 environment indicated by an increase in the polarisation resistance. However, with the introduction of acetic acid the corrosion rates increased significantly. The results of this study will form the basis of an understanding of the corrosion experienced at the top of the line and determining the amount of corrosion inhibitors to be injected using V-Jet™ pig in similar field environment to prevent carbon steel pipeline failure, and subsequent oil spillage.